Conventionally, suspension polymerization of vinyl compounds (for example, vinyl chloride) has been conducted to obtain vinyl resins from the vinyl compounds. It has been known to use a partially saponified vinyl alcohol polymer (hereinafter, a vinyl alcohol polymer is sometimes abbreviated as “PVA”) as a dispersion stabilizer for suspension polymerization of vinyl compounds.
The requirements for a dispersion stabilizer for suspension polymerization of vinyl compounds include: (1) even if the amount of the dispersion stabilizer for suspension polymerization used is small, the resulting vinyl resin has high plasticizer absorptivity and is easy to work with; (2) it is easy to remove residual monomer components from the resulting vinyl resin; (3) the resulting vinyl resin contains fewer coarse particles; and (4) the dispersion stabilizer for suspension polymerization has high water solubility and thus good handleability.
Demand for dispersion stabilizers that are more effective in meeting these requirements (1) to (4) is increasing more and more. In particular, the level of the above requirement (2) for easy removal of monomer components is very high, for example, in the production of hard polyvinyl chloride in which polymerization is conducted at a temperature of 60° C. or higher. Specifically, in the production of hard polyvinyl chloride at a polymerization temperature of 60° C. or higher, it is generally more difficult to remove residual vinyl chloride monomers in polyvinyl chloride particles in the drying step after the completion of polymerization, than in the production of hard polyvinyl chloride at a polymerization temperature lower than 60° C., and thus in the former case, drying needs to be carried out under more severe conditions or for a longer time to remove residual monomers. Therefore, there is a strong need for energy reduction and time reduction in the step of removing residual monomers, and thus there is a demand for a dispersion stabilizer that allows more residual monomers to be removed in a shorter time.
Even if a traditionally-used common partially saponified PVA is used as a dispersion stabilizer for suspension polymerization to respond to these increasing demands for more effective stabilizers, it is rather difficult for such a common PVA to fully meet these enhanced requirements.
Patent Literatures 1 and 2 each disclose a method for making a partially saponified PVA more effective as a dispersion stabilizer for suspension polymerization and propose the use of a PVA having a terminal alkyl group for suspension polymerization of a vinyl compound. However, in this method, if the degree of saponification and degree of polymerization of the PVA are low, the PVA is insoluble in water and cannot meet the requirement (4), although it is effective to some extent in meeting the above requirements (1) and (2). The water solubility is improved by increasing the degree of saponification, but the requirements (1) and (2) cannot be met instead. In view of this, Patent Literatures 3 and 4 each propose a method of using, as a dispersion stabilizer for suspension polymerization of a vinyl compound, a PVA obtained by introducing an oxyalkylene group into a PVA having a low degree of saponification and a low degree of polymerization to impart water solubility to the PVA. However, with respect to the requirements (1) to (4), this is not necessarily a good way to obtain satisfactory results. Furthermore, Patent Literatures 5 to 7 each propose a method of using, as a dispersion stabilizer for suspension polymerization of a vinyl compound, a PVA having an ionic group in its side chain. However, with respect to the requirements (1) to (3), this is also not necessarily a good way to obtain satisfactory results, although improvement is seen in the effect on the requirement (4).
Thus, it can be said that no dispersion stabilizer for suspension polymerization of vinyl compounds that fully meet these enhanced requirements (1) to (4) has been found up to now. Nor can various PVA-based dispersion stabilizers for suspension polymerization disclosed in Patent Literatures 1 to 7 fully meet these requirements.